238 ALTERNATE GENERATION AND 



the sexual animals arc formed, which separate from the stalk, swim 

 about indepeudeutly, and deposit their eggs in remote spots. 



In other hydroids the nurse acquires a still greater importance. In 

 them, as in our sweet-water polypi, the sexual progeny appears only in 

 the shape of globular appendages, which are not capable of being 

 evolved into independent animals, but remain attached to the polypus- 

 stalk, and resemble organs for the production of the sexual secretions. 



We may with Gegenbauer call this latter form of alternate generation 

 imperfect metagenesis. We see another remarkable instance of it in 

 the peculiar many-shaped colonies known as Siphoiiophorw, which swim 

 about freely in the sea, and of which the vraya dipheys, Blaine, occurring 

 in the Atlantic and the Mediterranean, may serve as an example. From 

 the transparent ovum of this animal a ciliated larva is hatched. The 

 plastic material contained in the body of this larva or nurse is then differ- 

 entiated into a locomoiory piece, (the posterior of the two swimming- 

 bells at the beginning of the stalk of a ripe colony,) and an appendage 

 whi(,'h afterward becomes the second bell and the common stalk of the 

 whole colony. The individuals now bud forth from this stalk in a fixed 

 order, but do not separate. They remain so connected that their abdom- 

 inal cavities all open into the canal passing through the common stalk. 

 These individuals are not by any means formed alike, nor do they serve 

 the same physiological purpose. The principal of the division of labor, 

 which is carried out in the solitary animals so that their organs become 

 constantly more numerous and more i)erfect, is here applied in such a 

 manner that the various functions of animal life, motion, alimentation, 

 defense, and aggression, as well as sexual reproduction, which is other- 

 wise confined to single individuals, are here distributed among all the 

 animals of the whole colony. In every tuft along the stalk, which some- 

 times numbers as many as fifty of them, we distinguish mnirisliers in the 

 form of trumpet-shaped appendages with orifices called suction- tubes ; 

 aggressors, in the form of long contractile filaments or tentacles furnished 

 with microscopic weapons (nettle-cells) at their knobs ; defenders, in 

 the form of stiff scales or helmets attached to the nourishers for pur- 

 poses of defense ; reproducers, developed after all the rest, in the form of 

 racemous diajcious ca])sules swinging in small (special) swiinming-bells. 

 By the alternate contraction and expansion of the bell-shiiped sicimmers 

 at the upper end of the colony, (the base,) with which the smaller spe- 

 cial swinnning-bells move in time, the whole colony is propelled tlirough 

 the water. 



In a few other species, the pliysalida; and vcUelida', the sexual ani- 

 mals separate from their nursing stalk and have a short, independent 

 existence like the medusa. 



The alternate generation of some of the intestinal worms is attended 

 by the most wonderful and extraordinary circumstances. The most 

 curious opinions have prevailed until very lately about their origin and 

 reproduction. • 



